Reference no: EM132186823
Introduction
In this Assignment you will undertake a design exercise for a commercial building. Data are provided from which you are to design parts of the electrical distribution. The building will have a public network supply with the following declared supply characteristics:
300 Ampere, 400/230 volt, TP&N, 50 Hz Prospective short circuit current: 25 kA TNCS, Earth fault loop impedance: 0.15 Ω
The incoming cable will terminate onto a main switchboard located on the ground floor. The Roof Distribution Board is fed from the Roof Plantroom Switchboard, but all other loads are fed radially from the main switchboard. The circuit details are shown in Table 1.
The following assumptions are to be made:
(i) All loads are three phase and operate at unity power factor.
(ii) The ambient temperature is 30°C throughout.
(iii) Cables are not installed in contact with any thermal insulation.
You will also have to make assumptions about the layout of the distribution, and choose the cabling system.
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Table 1 - Circuit Details
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Circuit Ref.
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Description
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Route Length (m)
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Design Current (A)
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Protective Device Rating (A)
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1
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IT server room D.B.
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100
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80
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160
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2
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Ground Floor D.B.
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70
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30
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63
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3
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First Floor D.B.(East)
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18
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20
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63
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4
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First Floor D.B. (West)
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73
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30
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63
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5
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Lift Motor Room D.B.
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85
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55
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63
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6
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Roof Plantroom Switchboard
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95
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100
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125
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7
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Roof D.B. (fed from the Roof Plantroom Switchboard, ref 6)
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20
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40
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63
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The Assignment
1. (a) Why do we impose an upper limit on the earth fault loop impedance, ZS, for each circuit?
(b) Why are we required to calculate the value of prospective short circuit current at points in the distribution system?
(c) How do we ensure that the live conductors of a cable are protected against the effects of overload and short circuit?
(d) How do we ensure that the circuit protective conductor is protected against the effects of fault current?
(e) What limitations do we need to impose on the volt drop in electrical distribution design?
2. (a) Draw a single-line schematic diagram of the electrical distribution.
(b) To exploit locally available renewable energy, an embedded generator is to be installed in the future. Identify what should be considered in designing the connection to the electrical distribution.
3. (a) For each of the circuits ref. 5, 6, and 7 select the type of: circuit protective device;
cable type; and
circuit protective conductor.
(b) Calculate the cable sizes for circuits ref 5, 6 and 7, demonstrating how you have satisfied each of the requirements from question 1.
4. (a) If a standby generator were installed to support the main switchboard, how would your design be affected?
(b) If the load currents included harmonic components, how would your design be affected?